The present invention relates to a method of controlling an electronically commutatable motor having field windings that are activated, connected to and disconnected from a direct voltage supply in succession with a commutation frequency via semiconductor output stages using control signals. The sequential control signals overlap and at least one of the control signals is clocked in the overlap region using pulse width modulation, so that the current in the field windings increases in the rising edge and decreases in the falling edge. The control signals are changed as a function of a selectable setpoint of the rotational speed or the output of the motor.
A method of controlling a motor of this type is described in German Patent No. 39 40 569 and U.S. Pat. No. 5,712,539. In these references, the continuously increasing and/or decreasing commutation edges of the phase currents are achieved with low loss using a selected pattern. Activation of the field windings in this manner has the advantage that, through the clocking of the control signals in the overlap region, noise reduction as well as low switching losses are achieved. However, the permanent activation of the phases in the actual commutation phases (outside the rising and falling edges) produces significant difficulties in the event the motor is used to produce a higher speed or output range.
German Published Patent Application No. 195 00 900 describes continuing application of current at the instant of commutation to the preceding winding phase in the direction of commutation with controlled reduction of the current intensity, and application of current to the following winding phase in the direction of commutation. In this manner, a notch of the overall current of the winding phases and a speed notch at the instant of commutation may be avoided. The preceding winding phase in the direction of commutation, however, has current applied to it with the pulse width modulated using a decreasing pulse width ratio. Neither application of current to the subsequent winding phases nor a method of reducing the application of current to the preceding winding phases as a function of other performance quantities are discussed.
In U.S. Pat. No. 5,869,946, a method of controlling an electronically commutatable motor using additional PWM signals generated for all phases is described. According to this method, a trapezoidal shape of the phase current, and therefore a constant overall output, are achieved.
German Patent No. 31 07 623, U.S. Pat. No. 5,767,641, and U.S. Pat. No. 5,780,986 describe changing the current or the voltage for the windings of a brushless direct current motor in the rising and falling edges as a function of the existing load of the motor.
European Published Patent Application No. 831 580 shows an electronically commutatable motor in which the control signals for the windings are implemented as PWM signals, the pulse width ratio following a sinusoidal pattern. In this manner, noise generation may be reduced. However, such reduction occurs at the cost of the output of the motor.
It is an object of the present invention to implement an electronically commutatable motor that, using simple pulse modulated control signals, suppresses noise generation which is greatly reduced over the entire commutation range and over the entire speed or output range.
This object is achieved according to the present invention by using PWM signals as control signals that have an operating value which is a function of a selected setpoint. In the rising edges of the control signals, the pulse width ratio increases from a low initial value to this operating value in a number of m steps which is a function of the setpoint or actual value. In the falling edges of the control signals, the pulse width ratio decreases in a number of n steps starting from this operating value before the pulse width ratio drops back to the value zero. The number steps, n, is also selected as a function of the setpoint or actual value.
Through the additional adjustment of the control signals in the rising and falling edges of the control signals to the selected setpoint or the existing actual value of the speed or the output of the motor, significant reduction in noise is achieved over the entire speed or output range of the motor without an excessive increase in control outlay.
Since the pulse width ratio in the rising and/or falling edges is not kept constant, but is changed as a function of the setpoint and/or the actual value of the speed or the output, smooth switching on and off of the current is achieved in the entire operating range of the motor. A strong force effect on the poles and field windings of the stator no longer occurs, so that the noise level is always significantly lower. Since the change of the pulse width ratio is also made a function of the actual value of the motor speed or output, this applies over the entire speed range of the motor and the noise reduction is achieved both for the switching on procedure and for the switching off procedure for the application of current.
According to one embodiment, the m and n steps of the pulse width ratio in the rising edges and/or the falling edges of the control signals are maintained over at least one period of the clock frequency of the PWM signals. The rising and/or falling edges of the control signals may also have m and n steps of changing pulse width ratios which each extend over a larger number of periods of the PWM signals.
In an embodiment in which selected speed or output ranges of the motor are assigned different numbers m and n of steps of the pulse width ratio and the assigned number m and n of steps of the pulse width ratio is put into effect in the rising edge and/or the falling edge of the control signals according to the actual value of the speed or the output of the motor, the number m and n of steps in the rising and/or falling edges of the control signals may be assigned immediately using the detected actual value of the motor speed or output.
It is advantageous in this case if the different numbers m and n of steps may be represented using 4, 8, 16, etc., using the base 2 and integer powers 1, 2, 3, etc., in order to keep the outlay in the motor control unit small.
According to one embodiment, the change in the pulse width ratio may be performed, in such a way that, in the rising edge of the control signals, the pulse width ratio is initialized using an initial value which is selected through the operating value assigned to the setpoint divided by the number of steps of the pulse width ratio in the rising edge, and that, in the falling edge of the control signals, the final pulse width ratio is determined using a final turn-off value which is selected through the operating value assigned to the setpoint divided by the number of steps of the pulse width ratio in the falling edge, before the pulse width ratio drops to zero.
For stepped modification of the pulse width ratio in the m steps of the rising edges of the control signals, the pulse width ratio is selected to increase from step to step by the initial value. In the n steps of the falling edges of the control signals, the pulse width ratio is selected decreasing from step to step by the final turn-off value.
The initial value and/or the final turn-off value of the pulse width ratio may also be zero.
If the PWM control signals switch through the semiconductor output stages, the power loss in the semiconductor output stages is kept low.
If the falling edge of the control signals for a semiconductor output stage at least partially overlap the rising edge of the control signals of the following semiconductor output stage during commutation, and if the semiconductor output stages may be determined individually and independently from one another in the overlap region of the control signals, then the control signals for the semiconductor output stages are individual and independent in the overlap region.